1. Field of Invention
This invention is on phase change material (PCM) chemicals used in PCM devices to store or remove thermal energy. Applications include (1) walls, flooring, and tank devices used to moderate climates in buildings (2) food storage coolers or other types of coolers, (3) devices used to keep food warm, and (4) essentially any device used to keep a substance at a relatively constant temperature between xe2x88x9220xc2x0 C. and 150xc2x0 C. More specifically, this invention is on a composition of PCM chemicals largely comprised of fatty acid derivatives, a method for producing these PCM chemicals, and a method for using these PCM chemicals.
2. Description of Prior Art
The term xe2x80x9cphase change materialxe2x80x9d or PCM is known in the science as that class of materials that uses phase changes to absorb or release heat at a relatively constant temperature. Typically the phase changes are fusion (or melting) with an associated latent heat.
Advantages of PCM in climate control include:
1. Eliminating need of air conditioner or heater requirements during substantial portions of the year.
2. Shift electricity usage from prime time to non-prime time.
3. Reducing the size of air conditioners needed to provide cooling requirement.
4. Substantially expanding regions in which heat pumps are practical for heating in wintertime.
Commonly used PCMs include hydrated salts, eutectic salts, and paraffins.
Feldman et al (D. Feldman, D. Banu, D. Hawes. Solar Energy Materials and Solar Cells 36 (1995) 147-157) demonstrated that a mixture of 20% methyl palmitate and 80% methyl stearate provided a sharp solid-liquid phase transition at ambient temperature with a latent enthalpy similar to that of paraffinsxe2x80x94this mixture is one of many possible derivatives of fats and oils. Unfortunately, the highly refined methyl palmitate and methyl stearate are too costly to compete with paraffins. This premise provides the motivation for the present invention. The present invention is on a method of producing fat and oil derivatives for use as PCMs and is not limited to any composition. No previous publications or inventions describe a PCM synthesis process similar to this invention. The present invention is also a method for using these PCM chemicals.
Fatty-acid based PCM can be produced in the following categories:
1. Naturally occurring triglycerides,
2. Hydrates of acids of triglycerides and their mixtures,
3. Esters of the fatty acids of naturally occurring triglycerides,
4. Refined/synthesized triglyceride products produced by a combination of fractionation and transesterification processes,
5. Synthesized triglyceride products using hydrogenation (or dehydrogenation) and fractionation,
6. Synthesized triglyceride products using cis-trans isomerization and fractionation,
7. Synthesized fatty acid derivatives that have the desired freezing point temperatures,
8. Refined fatty acid hydrates that have the desired freezing point temperatures, and
9. Prepared mixtures produced by essentially any of the previous processing approaches with other chemicals (preferable cheap and non-toxic) to produce eutectic compositions with the desired freezing point temperature range.
Naturally Occurring Triglyceride PCM
The naturally occurring triglycerides are the least expensive of the categories of fatty-acid based PCM. These are produced by separating natural triglyceride mixtures based on the temperature at which the triglycerides freeze. For example, if a PCM effective at 35xc2x0 C. is desired, the fraction of triglycerides in beef tallow that freezes between 34xc2x0 C. and 36xc2x0 C. is isolated from the mixture of naturally occurring triglycerides.
Preferably separation/fractionation is performed by a filtration or centrifugation process with the triglycerides in a solvent or antisolvent. To achieve the desired fraction, the mixture is filtered/centrifuged at a higher and a lower temperature. The higher temperature will determine the upper temperature of the range of utility of the PCM while the lower filtration/centrifugation temperature will determine the lower temperature of the range of utility of the PCM. If an antisolvent is used, the upper and lower filtration/centrifugation temperatures will be similar to the upper and lower range limits of the PCM. If a solvent is used, freezing point depression will occur and the filtration/centrifugation temperatures will be lower than the range limits of the PCM.
A trial and error procedure can be used to identify the filtration/centrifugation temperatures leading to the desired range limits for the PCM. Optimization routines such as the golden rule method can be used to reduce the number of attempts necessary to identify the correct filtration/centrifugation temperatures. For the golden rule method when used to identify the higher filtration/centrifugation temperature, the dependent variable would be the upper PCM range limit and the independent variable would be the upper filtration/centrifugation temperature. A similar approach would be used to identify the lower PCM range limit. Freezing point depression theory can be used to provide starting points for solvent filtration/centrifugation temperatures. When using antisolvents, high activity coefficients limit the accuracy of freezing point depression theory.
The filtration/centrifugation procedure can be repeated on the filtrate, possibly multiple times, to maximize the amount of latent heat released by the PCM within the temperature range limits. Preferably the solvent or antisolvent is easy to remove from the filtrate PCM. The mass of antisolvent to mass of triglyceride prepared for filtration/centrifugation is preferably between 20:1 and 1:1. Preferred methods of removing the solvent/antisolvent from the PCM are vapor-liquid separation for solvents and liquidxe2x80x94liquid separation for antisolvents.
Alternative to filtration and centrifugation, other separation methods known in the science are also able to achieve the desired separation including but not limited to settling.
If the naturally occurring triglyceride has useful latent heat properties between the desired PCM temperature ranges, further purification/separation is not necessary. When PCM chemicals are prepared with the desired latent heat properties the can be used in PCM devices by methods known in the art.
Alternative to using naturally occurring mixtures of triglycerides, triglycerides such as soybean oil can be hydrogenated to increase the freezing points of those components that react with hydrogen. Possible triglycerides include any commonly available animal fats, animal greases, or vegetable oils.
Hydrates of Acids of Triglyceride and Their Mixtures
Naturally occurring triglycerides come in hundreds of combinations of fatty acids. Hydrolysis processes known in the science can be used to decompose the triglycerides in to glycerin and fatty acids with the variety of fatty acids being significantly less. For example, if a triglyceride contains 20 different types of fatty acids, 20 to the 3rd power (8000) different triglycerides can be formed.
By fully hydrolyzing triglycerides and mixture containing far fewer chemical components is formedxe2x80x94it is thus feasible to separate the acids by methods known in the science into essentially pure acids or at least acid mixtures of considerably less variety.
Due to phenomena described by freezing point depression theory, mixtures generally tend to release latent heats over a larger temperature range than pure components with pure components often referred to as having a melting point temperature rather than a melting point temperature range.
The fatty acids or fatty acid mixtures (created by vapor-liquid separation) of a few components can be directly used as PCM chemicals; however, the latent heat temperature ranges are set for the pure fatty acids and may not be in the desired range for the targeted PCM devices.
Fatty acid mixtures can be prepared for targeted temperature ranges by the method previously described for preparing triglyceride-based PCM chemicals.
Alternatively, for fatty acids, two other approaches are viable including (1) correlating vapor liquid separation temperatures with PCM chemical limit ranges and (2) adding water to fatty acids or fatty acid mixtures to form hydrates that would freeze at a relatively narrow temperature range that is different than that of the anhydrous fatty acids. A trial and error approach will work with these approaches to expand the number of useful temperature ranges for latent heat removal.
Unfortunately, even with these degrees of freedom, only a limited number of useful PCM chemical temperature ranges can be achieved. Furthermore, only a small fraction of the acid mixtures will be used in these products.
Esters of the Fatty Acids of Naturally Occurring Triglycerides
The use of esters of fatty acids creates the greatest versatility to create chemical having the desired milting point temperatures. Esters of fatty acids can be formed by a variety of methods known in the science including alcoholysis and hydrolysis followed by esterification. The advantage of this approach is that relatively pure components having targeted melting point temperatures can be synthesized.
For example, a multitude of esters of oleic acid can be formed by complete esterification with methanol, ethanol, propanol, butanol, isobutanol, pentanol, hexanol, cylcohexanol, phenol, ethylene glycol, glycerin, diethylene glycol, and many more. To a first approximation, the oleate esters formed with each of these esters will result in different melting point temperatures. Furthermore, mixtures of two of the esters have the potential to form mixtures having relatively narrow and useful melting point temperature ranges.
The available fatty acids for reaction are dependent upon the initial triglyceride. The wide variety of components that can and do react with acid groups is not limited to alcohols and includes but is not limited to amines and other nitrogen or sulfur containing compounds. The vast number relatively pure components that can be formed allow PCM chemicals to be produced that are useful at a variety of temperatures. Furthermore, different acids can react with different co-reactants for form chemicals having essentially the same melting point temperatures. Furthermore, mixtures of these products can be used to essentially use the entirety of the naturally occurring fatty acids. Hydrogenation and other chemical modifications can be performed on the fatty acids to, for example, convert oleic acid to stearic acid and thereby limit the chemical diversity of the fatty acids.
This invention is a method for producing phase change material (PCM) chemicals containing fatty acids or fatty-acid derivatives. These derivatives (1) are renewable, being produced by biomass or livestock such as cattle, (2) can be manufactured at low to moderate prices, and (3) can be manufactured in a variety of ways to produce PCM chemicals effective at several temperatures of interest in climate control and food maintenance.
The embodiments of this invention reduce the cost and increase yields for the conversion of naturally occurring fats and oils to PCM chemicals.